1. Field of the Invention
The present invention relates to an encoding apparatus and a decoding apparatus. More particularly, the present invention relates to an encoding apparatus and a decoding apparatus capable of reducing the amount of information in encoding an audio signal while maintaining the sound quality.
2. Description of the Related Art
A number of encoding methods and decoding methods for an audio signal containing a speech or music signal have been developed to date. Among others, a recent method in conformity with IS13818-7, which is internationally standardized by the ISO/IEC, is valued as a high sound-quality and efficient encoding method. This encoding method is called AAC. Recently, AAC has been adopted by the standard called MPEG4 to produce MPEG4-AAC having several extended functions over IS13818-7. An example of the encoding process of MPEG4-AAC is described in INFOMATIVE PART.
An encoding apparatus using a conventional encoding method will be described below.
FIG. 16 is a diagram showing a structure of a conventional encoding apparatus 1600. The encoding apparatus 1600 comprises a spectrum normalization section 1601, a spectrum amplification section 1602, a spectrum quantization section 1603, a Huffman encoding section 1604, and an encoded sequence transfer section 1605.
An audio discrete signal (PCM data) obtained by sampling an audio signal is converted from data on a time domain to frequency spectral data using an orthogonal transformation technique or the like by a time-to-frequency conversion section (not shown). The data on a time domain of an audio signal is discrete data with respect to time, while the frequency spectral data of the audio signal is discrete data with respect to frequency. The frequency spectral data of an audio signal is input to the spectrum normalization section 1601.
An audio signal is divided into a plurality of frequency bands. The spectrum normalization section 1601 receives a frequency spectral sequence which is frequency spectral data in one of the frequency bands, and normalizes the average value of the frequency spectral sequence, using a scale factor, into a specific range to generate a normalized spectral sequence represented by a floating point. A scale factor is, for example, a multiplier coefficient for a power of 2.
The spectrum amplification section 1602 receives the normalized spectral sequence, and corrects each value of the normalized spectral sequence into a value in the specific range using a correction gain to generate an amplified spectral sequence.
The spectrum quantization section 1603 receives the amplified spectral sequence, and quantizes the amplified spectral sequence using a predetermined conversion expression into a quantized spectral sequence. The spectrum quantization section 1603 rounds spectral data represented by a floating point to integer values in the case of quantization in the AAC format.
The Huffman encoding section 1604 converts the quantized spectral sequence to a Huffman code sequence.
The encoded sequence transfer section 1605 transfers a scale factor output from the spectrum normalization section 1601, a correction gain output from the spectrum amplification section 1602, and a Huffman code sequence output from the Huffman encoding section 1604 to an external apparatus 1608. The external apparatus 1608 is, for example, a recording medium or a decoding apparatus.
Recently, it is desired for the compression rate of an audio signal to be increased so as to reduce the amount of encoded information.
The information compression performance of the encoding apparatus 1600 depends on the Huffman encoding section 1604. In the encoding apparatus 1600, to obtain a high compression rate of an audio signal, i.e., a small amount of encoded information, the correction gain of the spectrum amplification section 1602 is controlled in such a manner as to reduce the values of a quantized spectral sequence, such that the amount of information encoded by the Huffman encoding section 1604 is reduced.
With such an operation, however, when the Huffman code sequence is decoded into a frequency spectrum, a very large number of values having a zero amplitude (quantized value) are generated, so that sound quality cannot be sufficiently secured.